1. Field of the Invention
The invention relates to side-group polymers containing side groups whose configuration can be changed induced by light and permanently shape-anisotropic side groups, and to the use of these side-group polymers for optical components, in particular in optical data storage and transfer. Before irradiation, the novel side-group polymers in the glass state of the polymers are optically isotropic, amorphous, transparent and do not scatter light. Irradiation in the glass state causes, induced by light, an alignment of the side groups, causing the side-group polymers to become birefringent and dichroic without losing their transparency; in addition, the light scattering properties change. The light-induced optical anisotropy can be reversibly modified or eliminated thermally or by re-irradiation.
The side-group polymers are suitable for the storage of optical information and for the production of passive or optically switchable components.
2. Description of the Related Art
The literature discloses various polymers containing photochromic groups whose optical properties, such as absorption, emission, reflexion, birefringence or scattering, are reversibly modified by means of photoinduced physical and/or chemical processes. An interesting and novel method for reversibly modifying the optical properties of polymers is the photoinduced variation of the degree of ordering, the alignment direction and the alignment distribution or the morphology of the polymers by various processes. Photoinduced modification of the ordering in the polymers is used for storage of information.
Thus, Eich and Wendorff (Makromol. Chem., Rapid Commun. (1987) 8, 467) describe liquid-crystalline polymers containing azobenzene groups. These form liquid-crystalline domains in which the action of light only realigns those groups which have previously undergone an isomerization reaction (Anderle, Birenheide, Wendorff, Makromol. Chem., Macromol. Symp. 44, 11-22 (1991)). The storage effect is based on the interference in the ordered state of the monodomains by the realignment of the photochromic side groups in an otherwise rigid matrix.
In addition, liquid-crystalline polymers have recently been disclosed in which photochemical realignment of the photochromic groups also causes cooperative realignment of non-photochromic groups, so that the optical axis of the monodomains is rotated (Ivanov, Yakovlev, Kostromin, Shibaev, Lasker, Stumpe, Kreysig, Makromol. Chem., Rapid Commun. 12, 709-715 (1991)).
The severe applicational disadvantage of the liquid-crystalline systems is that their use generally requires a perfect, macroscopically uniform alignment by external fields and/or surface effects to give a monodomain.
The alignment of liquid-crystalline polymers by electric and magnetic fields and by mechanical forces and surface effects is extremely complex technologically, so that broad application of such liquid-crystalline polymers has hitherto not been possible.
It has furthermore been found that birefringence can in principle be induced in an amorphous system (Anderle, Birenheide, Eich, Wendorff, Makromol. Chem., Rapid, Comm. (1989) 10, 477ff and EP 335 302). However, the effect is some orders of magnitude weaker than that obtained from a liquid-crystalline monodomain and for this reason cannot be used industrially.
It is furthermore known (Natansohn, Rochon, Gosselin, Xie, Macromolecules 25, 2268-2273 (1992)) that certain homopolymers can form anisotropic properties on irradiation with light.
A system of this type is structurally invariant, is not stable in the long term and is fixed in its properties within broad limits. A particular advantage of copolymers over homopolymers is precisely their structural flexibility, which allows adaptation of the properties to the particular application with retention of the information-storing properties.
An advantageous system for the utility of photoaddressable materials in industry would be one which is on the one hand optically isotropic, amorphous and homogeneous and on the other hand has the excellent dichroic and birefringent properties of a liquid-crystalline polymer, in combination with a certain bandwidth in the structure variation in order to be able to match the other properties of the material to the particular technical requirements.